Arylalkyne Semi-Hydrogenation by Photoredox Catalysis

<jats:p>The semi‐hydrogenation of alkynes plays a crucial role in multistep organic synthesis and medicinal chemistry. Conventional hydrogenation relies on expensive and toxic heterogeneous catalysts based on scarce transition metals. These drawbacks highlight the importance of new sustainable and cost-effective alternatives. A new metal-free photoredox catalytic semi-hydrogenation of arylalkynes is reported by using a strongly reducing N-phenylphenothiazine or N-biphenylphenothiazine as organophotoredox catalyst and sterically hindered alcohols as hydrogen source. This simple protocol offers easy accessibility to a wide variety of arylalkenes as hydrogenation products. Comprehensive mechanistic studies combined with time-resolved spectroelectrochemical analysis and (TD)-DFT calculations elucidated the reaction mechanism. Hydrogen is transferred from the alcohol yielding a synthetic method with high selectivity, mild reaction conditions and simplicity. Overall, this is the first organo-photoredox catalytic reaction for Z-selective alkyne semihydrogenation and provides a powerful tool with tolerance to a wide variety of functional groups.</jats:p>